The use of air moving over a loudspeaker driver to cool the speaker and a driver assembly is well known in the prior art. Such speaker driver cooling systems have been in use for a number of years. Typically, air moving through a vent or a port in the speaker enclosure passes over the speaker driver assembly, thereby providing cooling. Recently, a powered speaker has been introduced that uses a diecast aluminum front panel in which are exposed vents or ports. Air moving through the vents flows past a heat sink, such as internal webs or fins, that provides an increased surface area by which means heat produced from the speaker is absorbed and/or stored or removed. The air flow past the heat sink provides for more efficient cooling of the electronic amplifier and other components within the panel, as well as the speaker disposed inside the enclosure.
In most prior art powered speakers, the power amplifier module is convection cooled because forced air cooling creates air and mechanical bearing noise, which is objectionable in a speaker system. Aside from the recent prior art speaker noted above, use of relatively high velocity air moving through the vent or port of a speaker enclosure has only been used for cooling the speaker's electromagnetic driver--not the electronic components used in a powered speaker. Previous approaches used for cooling the electronic amplifiers and other circuit components in a loudspeaker have included either simple flat panels made of aluminum plate, finned aluminum extrusions (such as shown in FIG. 12), or elaborate die-castings. However, all of these approaches have relied on convection cooling. Each implementation used in the prior art has its own strengths and weaknesses. Any blockage of the cooling airflow will cause the electronic circuitry and speaker driver to either run at unacceptably hot temperatures, or activate a thermal limit switch, de-energizing the speaker.
A common challenge in designing small powered speaker systems of one cubic foot or less in volume is finding sufficient space and surface area for mounting the electronic circuits and their associated heat sink. To develop sufficient acoustic power at low frequencies in a small enclosure, use of power amplifiers capable of operating at up to 350 watts are not uncommon. Due to the requirement for high power at extended low frequencies, alternative methods for cooling the power amplifiers and electronics package must be developed that are cost effective and functionally efficient.
A typical prior art powered speaker enclosure 2 is illustrated in FIGS. 10 and 12. An extruded heat sink 4 (FIG. 12), input connector 6 (FIG. 10) and power connector 8 (also FIG. 10) extend outwardly from a rear panel of speaker enclosure 2 a distance d.sub.2 (FIG. 10). As would be clearly noted in FIG. 10, the power and input connectors connect to a control panel 9 of FIG. 12, thus connectors 6 and 8 limit the proximity of speaker enclosure 2 to a wall or other surface behind the enclosure.
As shown in FIG. 10, the depth d.sub.1 of the speaker enclosure 2, which is typically eight to twelve inches for a speaker ranging in size of six to ten inches in diameter, includes an additional depth d.sub.2, which is the distance the connectors extend outwardly from the rear part of the speaker enclosure 2 approximately two to three inches. The overall enclosure depth d.sub.1 plus d.sub.2 will not always fit on a standard bookshelf. This is because the standard U.S. or European bookshelf is only eleven to twelve inches in depth. Although the overall depth of a speaker enclosure is not a performance issue, it is problematic to marketing considerations because a "bookshelf" speaker should be sized to actually fit on a standard bookshelf.
FIG. 4 shows another prior art enclosure 10 in which a vent 12 is disposed on the front panel of the enclosure, below a speaker 14. Sound waves emanate from speaker 14 and are designated as "17". Alternatively, vent 12' can be disposed on the rear panel 18 of enclosure 10, of which sound waves 17 will then radiate away from the rear of the enclosure, as shown in FIG. 5. A common problem with the configuration shown in FIG. 5 is that the rear facing vent 12' can be blocked when the rear panel 18 of enclosure 10 is placed against a wall 19 or other surface, such as shown in FIG. 6. In the graph of FIG. 7, a line 20 indicates the frequency response of a powered speaker having a vent that is not blocked in comparison to a line 22 that indicates the reduced low frequency response of the speaker when the vent is blocked.
In powered speakers, the space required for a heat sink 4 (FIG. 12) will conflict or interfere with that required for the electronic package having power amplifiers and/or other circuitry. If the electronic package is placed in or adjacent a rear facing vent to benefit from the air movement therethrough, and if the vent becomes blocked by placing a rear of the enclosure against a wall, overheating of the electronic circuitry will likely result, which can damage the speaker or, worse, cause an electrical fire.
Since it is desirable to maintain the smallest front panel surface area on a speaker enclosure for physical and acoustic reasons, while also providing the maximum amount of low frequency output, a passive radiator such as the kind disclosed in Applicants' co-pending U.S. patent application Ser. No. 09/115,507, filed Jul. 17, 1998, and entitled "Pistonic Motion, Large Excursion Passive Radiator" and which is incorporated by reference, is preferable to a vent, although more expensive. To achieve this small surface area on the front of the enclosure, the only surfaces available for mounting a passive radiator are the side panels, top panel, bottom panel and rear panel of the speaker enclosure. However, mounting the passive radiator on the side panels or top or bottom panels of the speaker enclosure is not practical because a user may lay the enclosure on its side or invert it, or suspend it from the ceiling so that the top is too close to the ceiling to permit the passive radiator to function properly. Thus, the only logical and practical place to locate the passive radiator is on the back of the enclosure. However, since the passive radiator discussed above occupies over 75% of the available surface area of the rear panel of the enclosure, there would not be sufficient room on the back of the enclosure to also include a solid extruded heat sink 4 like that used for the ported enclosure of FIG. 12.
Therefore, a novel approach is required that addresses each of the issues noted above. Specifically, the approach should enable a passive radiator and an appropriate cooling mechanism for an electronics package used in a powered speaker to be provided in a minimum amount of space and at a relatively low cost. The passive radiator and electronics package should both fit on the rear panel of a speaker enclosure and should not be subject to blockage when the enclosure is placed against a wall or other surface. The prior art does not provide a solution that meets these needs.